For example, according to the technology of layered coding and decoding speech signals, if encoded data of a certain layer is lost, it is possible to decode speech signals from encoded data of another layer. This encoding technology is typically referred to as “scalable speech coding” (or “scalable speech codec”). In scalable speech coding, there is also the speech coding referred to as band scalable speech coding. In band scalable speech coding, a processing layer that performs coding and decoding for narrow band signals and a processing layer that performs coding and decoding for making narrowband signals high quality and broadband signals, are used. Hereinafter, the former processing layer will be referred to as the “core layer” and the latter processing layer will be referred to as the “enhancement layer”.
Band scalable speech coding is assumed to be applied to speech data communication over a communication network where, for example, a transmission band is not ensured and thereby encoded data can be partially dropped (hereinafter, the dropped coded data refers to “loss”) as a result of a loss or delay. In this case, a receiver may receive coded data of core layer and enhancement layer (core layer coded data and enhancement layer coded data), or only the core layer coded data.
A method of interpolating information lost on the channel is, for example, disclosed in non-patent document 1. The method disclosed in the document is used in speech codec employing the single layer CELP (Code Excited Linear Prediction). Further, the method performs interpolation for parameters required to synthesize speech when a loss of the speech frame occurs, based on the past information. Particularly, as for gain coefficients, the interpolated gain coefficients are calculated by monotone decrease function for the gain coefficients received successfully in the past.
Non-patent document 1: “AMR Speech Codec; Error Concealment of lost frames”, 3GPP TS26.091 V5.0.0, 2002-06.